Commodore 64: Cyberload F4 (Multiload) loader
Many Cyberload tapes contain a 4th loader (and format) if the game is a multi-loader,
Just to make life difficult there are at least 3 (found so far) variations in the formatting of multiload files...
As with all Cyberload file formats, the loader program itself should be located and interrogated to determine the used threshold, pilot and sync variables for any particular tape.
Threshold : varies between tapes (and between files for types 2 and 3)
Endianess : MSbF
Pilot byte : Varies beween tapes. (commonly $0F)
Sync byte : Varies between tapes. (commonly $AA, $96 or $99)
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The structure of the files is very similar to Novaload, Each is organised as a chain of small sub-blocks preceeded by a main header section, it is these header sections that can vary between tapes...
Format 4, Type 1 Header (found on The Last Ninja)...
1 byte : Sync byte
16 bytes : Filename
2 bytes : Load address (LSBF)
2 bytes : Data size (LSBF)
1 byte : Header checksum (XOR of filename, load address and data size)
n bytes : Data
Format 4, Type 2 Header (found on Ninja Spirit, Last Ninja 2)...
1 byte : Sync byte
16 bytes : Filename
2 bytes : Load address (LSBF)
2 bytes : Data size (LSBF)
2 bytes : Encrypted threshold to load data (LSBF)
1 byte : Header checksum (XOR of previous data, starting from filename)
1 byte : $00 closing byte (fake checksum)
n bytes : Data
Format 4, Type 3 Header (found on Last Ninja 3, Vendetta, Hammerfist)...
1 byte : Sync byte
1 byte : Flag byte
16 bytes : Filename
2 bytes : Load address (LSBF)
2 bytes : Data size (LSBF)
2 bytes : Enciphered pulsewidth threshold (LSBF)
1 byte : Header checksum (XOR of previous data, starting from filename)
1 byte : $00 closing byte (fake checksum)
n bytes : Data
After each header there begins a chain of sub-blocks with the length of each being 256 if there is some more data available otherwise the length is whatever is left, until <data size> data bytes are read in total.
Sub-blocks are written out consecutively without pauses or anything else between them, each one has the following structure...
n bytes : Data (either 256 or less bytes)
1 byte : XOR checksum of sub-block Data
After all sub-blocks there is a closing byte, $00, which may be thought of as a fake FINAL checksum, if you XOR all blocks and their own XOR checksums, the result should be $00 if the data is all ok.
Notes:
Deciphering threshold (F4.2 & F4.3)...
High byte is shifted RIGHT thus affecting Carry, then low byte is rotated right and, after clearing Carry, it is added to its starting value. Now, without clearing Carry, the previously shifted high byte is added to its starting value.
In other words... Threshold = Enciphered_Threshold * 1.5!.
Note: The F4 loader is usually stored in a 2nd F2 file immediately proceeding any F4 files.
(Exceptions to this rule are Afterburner and Alien 3 which should be investigated).
